Image transfer sheet and method

ABSTRACT

A copy sheet having a coating comprising a binder and a reactive water-insoluble heavy metal salt of an organic acid and capable of forming a visible image on application of a heat pattern in presence of a coreactant for said salt, the resulting image areas being physically separable from the remaining background areas in the absence of any temperature differential therebetween.

United States Patent Inventor Donald J. Williams White Bear Lake, Minn.

Appl. No. 789,392

Filed Jan. 6, 1969 Patented Jan. 4, 1972 Assignee Minnesota Mining and Manufacturing Company St. Paul, Minn.

IMAGE TRANSFER SHEET AND METHOD 4 Claims, No Drawings US. Cl 117/36.2, 117/36.8, 117/76 A, 101/469 Int. Cl B41rn 5/22 Field of Search l17/36.2, 36.8

References Cited UNITED STATES PATENTS Owen Owen

Newman Johnson et a].

Dulmage et a1 Haas Primary ExaminerMurray Katz Attorney-Kinney, Alexander, Sell, Steldt & Delahunt IMAGE TRANSFER SHEET AND METHOD This invention relates to copy sheet materials and to the art of recording or copying. In one aspect the invention relates to the stabilizing of records prepared on uniformly coated sheet materials by the separation of imaged and nonimaged areas of the coating. in another aspect the invention relates to the separation of the image areas from the copy sheet, and by separation processes requiring no preferential heating or other differential processing of said areas. In a further important aspect the invention relates to the transfer of image areas from the copy sheet to a separate receptor sheet. The invention also relates to the preparation of novel copy sheet materials capable of providing selectively transferable images. Thus the invention is useful in improving the stability of certain forms of records or copies, in transferring intelligence from graphic originals to desired receptor surfaces, in the preparation of projection transparencies and for other purposes.

The transfer of intelligence from a graphic original to a receptor surface finds particular application in the preparation and revision of engineering drawings, and the invention will be described primarily in terms of such operations in accordance with the principles of the invention, a copy of the desired portion of an original is first prepared by a process involving a heat-induced reaction, at image-defining areas within the coating of a coated transfer sheet, between a waterinsoluble organic acid salt of a heavy metal and a suitable coreactant therefor. The transfer sheet is then stripped away. The imaged portion of the coating offsets and remains adhered to the drawing; the remaining background portions are removed with the transfer sheet backing.

The process provides a number of advantages. The transfer image may be produced in any of a number of colors if desired. Reproduction and transfer of intelligence from any printed or other graphic source is possible, including duplication of segments of the same or other engineering drawings. The transferred image is firmly bonded to the tracing paper, vellum or other substrate forming the base for the drawing and is not disturbed by added pencil markings. Blueprint or diazo reproductions of the thus augmented drawing are free of the ghosting" obtained with paste-on additions.

Printed adherent transfer image sheet materials in the form of decals have previously been made available wherein a removable image is formed by printing with a film-forming ink or varnish on a release surface and then overcoating the area with a coating of adhesive which adheres firmly both to the release surface and to the printed image. The segment is pressed against a receptor and then stripped away, leaving the image adhered to the receptor. The user is limited to those preprinted images and in those colors which are available, so that an unduly large inventory must be maintained. The images thus transferred possess sufiicient integrity, due to the strength of the ink film, to be removed bodily from the receptor surface upon the passage of a draftsmans pencil or drafting instrument over the imaged area.

Copy sheets having reactive coatings comprising a water-insoluble organic acid salt of a heavy metal are well known. Such sheet materials ordinarily contain a film-forming polymeric binder and may include a coreactant for the metal salt, as in the therrnographic copy-sheets of Owen U.S. Pat. No. 2,9l0,377 or of Miller et al. U.S. Pat. No. 2,663,657; or the coreactant may be a light-desensitizable material obtained from a separate intermediate sheet, as in Workman U.S. Pat. No. 3,094,417 or in Reitter U.S. Pat. No. 3,094,620. Sheet materials of both types have been used in the modifying of engineering drawings, the imaged portion being simply pasted in place over the indicated area; but reproductions of such drawings prepared on light-sensitive blueprint or diazo paper show ghost images due to the increased thickness of the imaged segment.

The coatings and coated sheet materials of these several prior art patents are found to be ineffective in the processes and for the purposes of the present invention. In no instance, using the copy sheet structures as described in these several U.S. Pat. specifications, has it been found possible to achieve selective offsetting of imaged areas to a receptor sheet as herein described; and the patents do not teach or suggest any selective offsetting. Separation of image from background areas may be effected by selective heating of the image areas while in contact with the receptor sheet surface, as taught in Owen U.S. Pat. No. 3,315,598; but the present invention avoids the requirement of selective heating and provides for selective offsetting of image areas under uniform overall processing.

Opaque black images are normally desired on engineering drawings, and a preferred form of image transfer sheet therefore produces selectively transferable opaque black images, e.g. by reaction between a silver soap and a reducing agent as described in Owen U.S. Pat. No. 2,910,377 and in the presence of a toner for the silver image preferably as described in Grant U.S. Pat. No. 3,080,254. However in order to obtain the required selective transfer property it is essential to select appropriate backing and binder materials, to employ specific proportions of components, and in some instances to include additional or auxiliary components. Once the ability to undergo selective transfer of image has been imparted, it is then necessary additionally to select appropriate adhesive transfer agents and properly to apply and use the same, so that image transfer may indeed be accomplished. Similar considerations apply to the transfer of images of other colors and characteristics, for example those obtained from coatings containing nickel soaps and reacted with dithiooxamide or its derivatives to provide variously colored images with which color projection transparencies may be made.

The selection of the carrier sheet or backing from which the image is to be transferred is an important first step in preparing the novel sheet materials of this invention. In general, the best backings are dense papers of low porosity and high surface smoothness. A preferred example is map overlay tracing (MOT) paper; greaseproof paper, airmail bond, and glassine are also useful. The coating composition is applied to the backing from suspension or solution in a volatile liquid vehicle which is removed by evaporation. The ability of the imaged coating to offset from the carrier may be maximized by drying the coating as rapidly as possible, so that in many cases a marginal combination of carrier and coating may be made fully effective merely by increasing or decreasing the rate of coating and drying.

An essential component of the image-forming coating is the heavy metal salt of an organic acid. The heat-induced reaction between the salt and a coreactant which results in the formation of a visible image results also in the liberation of free or ganic acid; and it is this acid which is responsible for the ability of the image subsequently to be selectively offset from the carrier. For this purpose the long chain fatty acids having a carbon chain length of at least 10 carbon atoms are effective. Silver behenate, ferric stearate and nickel palmitate are typical of heavy metal salts which are useful in the preparation of image forming coatings and coated sheet materials of this invention.

A polymeric film forming binder material forms another important component of the coating. Lucite 2013, a copolymer of methyl methacrylate and isobutyl methacrylate, is particularly effective as a binder in compositions containing silver behenate. Polymethylmethacrylate, and mixtures thereof with up to one-half its weight of polyvinyl acetate, are also useful. Various other film-forming polymeric binders may or may not be effective; but the utility of such materials for the purposes of the invention may be determined by a simple tape test which will now be described. The same test may be used also in estimating the effectiveness of a particular carrier or backing with a given coating composition.

In making the test, thin coatings of the unfilled binder and of the binder filled with six times its weight of well-dispersed titanium dioxide pigment or equivalent inert filler are applied to the backing with a volatile liquid solvent. The two mixtures are each coated at a viscosity of approximately l00 c.p.s. (Brookfield), which typically requires a concentration of about 20 percent for the polymer and about 35 percent for the polymer plus filler, and are promptly dried at 200 F. The coating weights after drying are 0.6-0.7 grams per sq. ft. for the unfilled polymer and 0.70.8 gm./sq.ft. for the polymer plus filler. The dried coatings are heated to 250 F. for 20 seconds. Strips of pressure-sensitive adhesive tape are uniformly pressed onto the coated surfaces, using a weighted roller, and are then peeled away at constant slow speed. An adhesive tape is selected so that, under the same conditions of application and removal, the force required to remove the tape from the untreated backing is at least 400 gm. per inch of width. The details of application and removal of the tape are as described under the ASTM Keil test, D 903-49. The degree of offsetting of the two coatings from the paper to the adhesive tape surface measures the effectiveness of the binder. An effective binder is one which by itself does not offset, but which when combined with the pigment is removed from at least one-half, preferably all, of the test area.

The amount of heavy metal salt to be incorporated with the binder must be sufficient to produce a dense visible image when properly reacted, while still being adequately integrated and bonded by the binder. A favorable ratio in the case of silver behenate and polymethyl methacrylate is two parts of the former to from two to three parts of the latter, depending somewhat on the amounts of other materials incorporated in the coating. These concentrations of heavy metal salt are found also to make available an amount of fatty acid sufficient to provide the desired selective offsetting properties at the reacted image areas, providing the reaction is carried sufficiently toward completion. in the case of silver behenate reacted with a reducing agent, completion of the reaction as well as intensification of image density is readily achieved by the inclusion of phthalazinone.

The incorporation of free fatty acid in or beneath the original coating decreases the bond between coating and backing. Binders exhibiting unduly high bonding tendencies may therefore be modified by the addition of small amounts of such materials which then augment the effects of the reactionliberated fatty acid and make possible the selective offsetting of the image. The amounts of fatty acid which may be safely added for such purposes are critical, so that other means of control wili ordinarily be preferred. One such means involves the addition of substantial quantities of inert powder, of which powdered glass is a preferred example. A typical recipe may contain powdered glass, having a nominal particle size of about 5-l5 microns, in an amount equal to the weight of heavy metal salt.

A coreactant in amount sufficient to react with the entire quantity of heavy metal salt may be included with the salt in the coating. Image formation is then readily achieved by thermographic copying techniques involving brief exposure of a differentially radiation-absorptive graphic original to intense radiation while in heat-conductive contact with the coating. Conversely, the coreactant may be supplied from a separate source sheet using the same thermographic process to transfer the material to the coating and cause it to react with the metal salt. A preferred procedure employs a coating containing the metal salt and less than the full amount of coreactant needed for complete reaction, together with a separate source sheet containing a further coreactant which is photosensitive. The photosensitive sheet is first exposed to a light-pattern corresponding to the graphic original, usually by reflex exposure and to an extent just sufficient to desensitize the sheet at nonimage areas. It is then placed against the metal salt coating and the two are uniformly heated to cause transfer of the remaining coreactant and formation of the corresponding visible and selectively ofisetting image.

Once the image has been formed, it may be selectively off set in any of a number of ways. A segment of transparent pres sure-sensitive adhesive tape pressed onto the imaged area and then stripped away will remove and retain only the image, and may then be readhered to the surface of any desired receptor. Waxed paper is sufficiently adherent to the coating to cause selective offsetting of the imaged areas when applied by burnishing and then peeled from the surface. A thin adhesive coating or film applied over the imaged coating makes possible the selective transfer of the image to any suitable receptor surface, the sheet being first uniformly pressed or burnished against the receptor over the entire image-containing area and then peeled away to remove both adhesive and coating at the nonimage portions. Microcrystalline wax, pressuresensitive tape adhesive or preformed adhesive films of relatively low tackiness, and various special-purpose adhesive are all useful in the last-named procedure. Thermoplastic adhesives may also be used. Light-polymerizable adhesive compositions applied over the imaged coating and exposed to light through the backing are rendered nontacky at nonimage areas but remain tacky at image areas, and selective offsetting of the image to a receptor surface is readily accomplished by this route also.

In all cases, separation of image and nonimage areas pro vides stable positive and negative reproductions of the graphic original. The offset image forms a positive reproduction of the printed intelligence of the original. The residual nonimage portions define a negative reproduction and may be intensified by reacting the metal salt contained therein with a suitable coreactant; or a distinctly colored or light-transmitting backing may be used, in which case the nonimage areas of the coating serve as a mask or stencil.

The practice of the invention is further illustrated in the following nonlimiting specific examples wherein all proportions are in parts by weight unless otherwise indicated.

Mixture B 2,6-Ditertiarybutyl-A-methylphenol The materials are blended into a uniform mixture, coated on 30 lb. MOT paper using a coating bar set at an orifice of 4 mils, and dried at 200 F., to provide an image sheet. Preparation of Mixture A:

Mix 50 parts of silver behenate and 4 parts of tetrachlorophthalic anhydride into a solution of 25 parts of cellulose acetate in 225 parts of acetone, and pass through a homogenizer under a pressure of 8,000 psi. Add 75 parts of powdered glass (Min-U-Sil 15) and 200 parts of acetone, and homogenize at 6,000 psi. Add a solution of 7.5 parts phthalazinone and 50 parts of cellulose acetate in 400 parts of acetone, and mix well.

Preparation of Mixture B:

Mix 50 parts of silver behenate and 4 parts of tetrachlorophthalic anhydride into a solution of 25 parts of polyvinyl formal (Formvar" resin) in 225 parts of acetone, and homogenize at 8,000 psi. Add 50 parts of powdered glass and 200 parts of acetone, and homogenize at 6,000 psi. Add 7.5 parts of phthalazinone, and mix well.

The photosensitive intermediate is placed against a completed segment of an engineering drawing for exposure in reflex position to light from a bank of tungsten filament lamps. The exposure is just sufficient to desensitize the nonimage areas.

The exposed intermediate is placed against the image sheet and the composite is uniformly heated. An image corresponding to the original drawing is produced. The exhausted intermediate is discarded.

A strip of transparent pressure-sensitive adhesive tape is pressed onto a portion of the imaged sheet and then peeled away. The image remains bonded to the tape and offsets from the sheet, whereas the nonimaged portion of the coating remains with the MOT paper backing The tape bearing the image is adhered to the original drawing.

The same mixture of polyvinylformal and cellulose acetate is separately tested by the Keil test procedure hereinbefore described, with preliminary brief heating of the test strips to 250 F. prior to application of the tape. The unpigmented coating remains adhered to the MOT paper. The pigmented coating is completely removed. The same test is applied to the two polymers separately. The polyvinylformal remains adhered to the paper in both filled and unfilled condition, whereas the cellulose acetate is fully removed both filled and unfilled.

EXAMPLE 2 A first mixture is prepared by dissolving 12 parts of polymethyl methacrylate and 1.92 parts of tetrachlorophthalic anhydride in 362 parts of acetone, adding 24 parts of silver behenate and homogenizing at 8,000 p.s.i. adding 20 parts of "Min-U-Sil M-l5" ground glass and 80 parts of acetone, and homogenizing at 6,000 p.s.i. To this mixture is added a mixture of 3.62 parts of phthalazinone, 12 parts of polymethyl methacrylate, and 12 parts of polyvinyl acetate in 72 parts of acetone, with thorough mixing. Three parts of 2,6-ditertiarybutyl-4methylpenol are then added, with continued mixing, and the mixture is coated at four mils on 30 lb. MOT paper and dried at 200 F. for 2 minutes. The dry coating weight is 0.65 gm./sq.ft. The ground glass may be omitted where sufficiently prompt drying of the coating is achieved.

An intermediate sheet prepared and exposed as in example 1 is placed against the coated sheet and the composite is heated for about 20 seconds on a heating platen having a uniform temperature of 245 F. to develop a dense black image corresponding to that of the graphic original. The intermediate is removed and discarded.

A thin film of transparent pressure-sensitive tape adhesive of the rubber-resin type, supported on a temporary carrier of silicone-coated paper, is laid over the imaged portion of the coated sheet with sufficient pressure to permit transfer of the adhesive thereto, and the composite is heated on the heating platen for about -15 seconds to anchor the adhesive to the coated copy sheet. The carrier sheet is removed. The copy sheet is laid in position on an engineering drawing and is rubbed briskly with a smooth-surfaced burnishing tool uniformly over the imaged area. The copy sheet is then carefully peeled away. The image remains adhered to the drawing paper; the remainder of the coating, with the adhesive film adhered thereto, is removed with the copy sheet backing.

Similar results are obtained by substituting a thin coating of microcrystalline wax for the pressure-sensitive adhesive film. The wax is applied in molten liquid form at about 175200 F. to the coated surface of the imaged copy sheet, and permitted to cool and solidify. The sheet is placed against the drawing, burnished uniformly, and peeled away. The image selectively offsets and remains as a part of the drawing.

The binder mixture used in this example remains adhered to MOT paper under the tape test, whereas the pigmented binder coating is completely removed.

EXAMPLE 3 The copy sheet of example 2 is provided with an image as there described. An adhesive film is separately prepared by coating a silicone-treated paper carrier with a solution in acetone of an adhesive consisting of 95 parts of a photosensitive reaction product of hydroxybutyl vinyl ether and toluene diisocyanate, 5 parts of polyoctylacrylate, and 5 parts of tetrabromo-o-xylene. The coating is dried and the sheet is pressed against the imaged copy sheet surface. The composite is then exposed from the back surface of the copy sheet to ultraviolet light for a time just sufficient to destroy the tackiness of the adhesive layer in the nonimage areas. The areas protected by the opaque image remain tacky. The carrier is removed and sheet placed against an engineering drawing, burnished uniformly over the imaged portion, and then peeled away, leaving the image as a part of the drawing.

EXAMPLE 4 Thirty pound MOT paper is coated at a wet thickness of 3 mils with a homogenized mixture of 24 parts of polymethyl methacrylate, 48 parts of ferric stearate, and 40 parts of ground glass in 288 parts of acetone, and dried at 200 F., to form a copy sheet.

Thin porous paper is sparingly coated with a coating containing 20 parts of methyl gallate, 20 parts of cornstarch, and one part of polyvinyl acetate, applied from a mixture of methylisobutyl ketone and alcohol, to provide an intermediate sheet. indicia are applied to the uncoated surface by printing with a typewriter and by writing with a pencil. The intermediate is placed against the copy sheet and while in that position is briefly exposed to intense infrared radiation, causing corresponding indicia to appear on the copy sheet. The intermediate is removed and discarded. The indicia on the copy sheet are transferred to another receptor surface by means of a strip of transparent pressure-sensitive adhesive tape as described under example 1.

In the tape test, unpigmented polymethyl methacrylate remains bonded to MOT paper whereas the same binder containing six times its weight of titanium dioxide pigment is found to undergo complete offsetting.

EXAMPLE 5 A copy sheet is prepared by coating MOT paper with a homogenized mixture of 48 parts of nickel stearate, 40 parts of ground glass, 24 parts of polymethyl methacrylate, and 400 parts of acetone. The coating is applied at a thickness of 4 mils, and dried.

An intermediate is prepared by applying to thin porous paper a light coating of a mixture of difurfuryl dithiooxamide and a minor amount of ethyl cellulose. The intermediate is placed against the copy sheet. A graphic original, having in frared-absorptive indicia on a thin white paper, is placed against the exposed surface of the intermediate and while in that position the printed surface is briefly exposed to intense infrared radiation. A copy of the indicia is formed at the surface of the copy sheet, and is found to be capable of selective offsetting. The copy is transferred to an engineering drawing by the methods of the previous examples.

Transfer of images from the imaged copy sheet of this exam ple to a transparent receptor film or plate makes available a color projection transparency. Other image colors may be provided using intermediates containing other coreactants, of which dimethyl dithiooxamide is one example.

EXAMPLE 6 A copy sheet having a transparent backing is advantageous where precise placement of the image on the receptor surface is desired. A suitable backing for such purposes is prepared by coating a transparent Mylar" oriented polyethylene terephthalate film with a mixture of 2.5 parts of polyethylene terephthalate-isophthalate (9:1) polymer and 5 parts of ground glass, applied at 3 mils and dried, followed by a 2 -mil coat of an aqueous 5 percent solution of hydroxyethyl cellulose and again drying. A copy sheet coating as used in example 2 exhibits selective offsetting of image areas from such a film surface, whereas the entire coating will offset from the untreated polyethylene terephthalate polyester film. The mixture of two parts of polymethyl methacrylate and one part of polyvinyl acetate used as the binder in the copy sheet of example 2 passes the tape test when applied to the subbed film whereas both the pigmented and the unpigmented binder coating offsets from the original Mylar polyester film.

EXAMPLE 7 Thirty pound MOT paper is coated at a coater bar setting of three mils with a homogenized mixture of 12.5 parts of ferric stearate, 6.5 parts of polymethyl methacrylate, 12.5 parts of ground glass, 2.5 parts of methyl gallate, and 66.25 parts of acetone. The coating is dried in a current of air at 200 F. The sheet is placed with the uncoated surface against a printed original and the composite is passed through a thermographic copying machine to form a copy of the printed indicia by front printing". The image areas are found to be capable of selective offsetting, using a transparent pressure-sensitive adhesive tape as described under example 1, with formation of a positive copy. The remaining copy sheet with image areas removed is heated to an extent sufficient to cause reaction and darkening of the nonimage areas, thereby forming a stable negative copy.

The addition of ground glass has been shown to improve the ability of the imaged portions to undergo offsetting. Other inert fillers useful for the same purpose include asbcstine, powdered alumina, silica powder, powdered quartz. Titanium dioxide pigment is also useful where its photosensitivity does not cause unwanted reactivity, as in compositions containing silver salts.

Transfer or offsetting of oleophilic images to sheet materials having a normally hydrophilic surface by methods as described hereinabove provides master plates useful for lithographic printing. Incorporation within the copy sheet coating of soluble or volatilizable dyes or reactants makes possible the preparation, by image offset and transfer to suitable receptor sheets, of imaged masters useful in the preparation of additional copies by spirit offset or by vapor transfer techniques. Transfer of image to a metal receptor, e.g. using an acid-resistant transfer adhesive as in example 2, provides an etch resist which permits selective etching of the metal receptor in the preparation of intaglio printing plates or printed circuit boards. Other applications will occur to those skilled in the art in view of the disclosures and teachings herein contained.

What is claimed is as follows:

1. The method of imparting indicia to a receptor surface, comprising the steps of: producing said indicia, in the coating of a transfer sheet having a dense smooth-surfaced flexible backing and a coating thereon comprising a polymeric filmforming binder, a heavy metal salt of a long chain fatty acid, and an inert light-stable inorganic powder, the ratio of said powder to said salt being between 10:5 and It): I 8, by heat-induced reaction between said salt and a coreactant therefor to form a visible image and to release free long chain fatty acid from said metal salt; applying over the imaged coating a thin film of adhesive, said adhesive being more adherent to the imaged coating than to said receptor surface; pressing the adhesive-coated sheet against a receptor surface; and stripping said adhesive-coated transfer sheet from said receptor. leaving said image adhered to said receptor by means of its overlying adhesive.

2. The method of claim I wherein said metal salt is a silver salt and wherein said coreactant is a photosensitive substituted alpha-naphthol.

3. The method of claim 1 wherein said coreactant is applied to said coating from an intermediate sheet by heating.

4. The method of claim 1 wherein said metal salt is a salt of silver, iron or nickel and said coreactant is included in said coating.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 3 ,377 7 Dated January L 1972 Inventor(s) Donald J. Williams It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 27, after "therefor," the following sentence has been omitted:

That portion of theitransfer sheet is adherently attached to the engineeringdrawing, withthe image in the desired position."

Col. 4, line 57, the following paragraph has been omitted:

"A photosensitive intermediate sheet is separately prepared by coating thin transparent polyester film with a solution of 0.2 part of L-methoxy--l-n aphthol, 0.088 part of erythrosin, and lO parts of ethyl cellulose in 90 parts of methylethyl ketone, applied at a wet thickness of 3 mils and dried, with care to avoid exposure to light."

001. 6, line 70, "6.5" should be "6.25"

Signed and sealed this 11 th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOI'TSCHALK A1Ltesiing Officer Commissioner of Patents ORM PO-105D (10-69) USCOMM-DC 60376-F'69 US. GOVERNMENT PRINTING OFFICE: I969 O-QGS-JSI 

2. The method of claim 1 wherein said metal salt is a silver salt and wherein said coreactant is a photosensitive substituted alpha-naphthol.
 3. The method of claim 1 wherein said coreactant is applied to said coating from an intermediate sheet by heating.
 4. The method of claim 1 wherein said metal salt is a salt of silver, iron or nickel and said coreactant is included in said coating. 